1. Field
This present application generally relates to a document reading apparatus for document readers, copy machines, facsimile machines and other machines used to replicate an original image. More particularly, the present application relates to a document reading apparatus having a white platen roller which is used for shading correction.
2. Description of the Related Art
Document reading apparatus used, for example, in copy machines, facsimile machines and document readers (or scanners), typically obtain a reference measurement prior to scanning one or more original documents in order to compensate for shading variations in the original documents. This compensating process is known as shading correction.
One type of conventional document reading apparatus which has a contact-type image sensor and provides shading correction is disclosed in Japanese Laid-Open Patent Application No.4-183166. This apparatus has a white platen roller which makes contact with the contact-type image sensor at a document reading position. The document is read when it is transported in a state pinched between the contact-type image sensor and the white platen roller.
One drawback to such document reading apparatus is that the outer peripheral surface of the white platen roller becomes dirty due to contact between the roller and the document. That is, the white platen roller collects dust or other debris due to the contact between the roller and documents. As a result, if the contact-type image sensor reads the dirty portion of the white platen roller it is difficult to carry out accurate shading correction.
Another type of document reading apparatus that provides shading correction is described in Japanese Laid-Open Patent Application No.2-70170. This apparatus also uses a white platen roller and a contact-type image sensor. In this document reading apparatus the white platen roller is rotated to different positions and at each position a reference white signal is measured by the image sensor. The output of the contact-type image sensor (i.e., each reference white signal) is stored and compared to each previously stored reference white signal by a comparison circuit. If a newly measured reference white signal is greater than the stored one, the new reference white signal is stored. Thus, a maximum reference white signal is maintained in memory, and the maximum reference white signal is used for shading correction.
Typically, the white platen rollers are manufactured with metallic molds 101a and 101b, seen in FIG. 1b, which when joined create crevices `M`. These crevices create burrs on an outer peripheral surface of the rollers, as shown in FIG. 1a. The burrs `C` are typically created on opposite sides of the circumference of the white platen roller 100.
If the contact-type image sensor reads near a burr on the white platen roller, the output of the image sensor will reflect a problem that is similar to the case where dirt on the white platen roller is read, which may result in inaccurate shading correction.
Further, most document reading apparatus use a motor is used to move or rotate the white platen roller. When the motor is actuated electrical noise is typically generated and radiates within the document reading apparatus. Therefore, if the contact-type image sensor reads the white platen roller while the motor is energized, the electrical noise may mix with output of the image sensor and affect the white reference data which may result in inaccurate shading correction.